If I were going to do it, I would just put the hose in the top of the carboy and cover it all with foil. It's not gonna be on there for long, you don't want to over pitch the receiving carboy either. I'd say an hour would be more than enough.

For a bucket I wouldn't do this. You'd be better to top crop and be done with it in 5 minutes.

How did you release the pressure? I'm picturing a blowoff tube from one grommet to another. If that is the only outlet for each, how did you account for the change in pressure going in to the underpitched batch? Or am I missing something obvious? Happens to me sometimes.....

Carboy caps would work, methinks. Airlock in one hole and a tube in the other?

That makes sense. There has to be a way to relieve the pressure from the combined system.

Gonna keg this today and keep some for re-pitching. Based on this, would it be better to keep the yeast from the underpitched or the over pitched for a porter (WLP007). Gonna keg today and brew tomorrow.Will update with FG and comments later.

I would say overpitched but can't wait to hear other responses on that.